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(16)Aromatic Compounds: Comprehensive Notes

Organic Chemistry: Aromatic Compounds

Discovery of Benzene

  • Benzene was first isolated in 1825 by Michael Faraday.
    • He determined the carbon to hydrogen ratio to be 1:1.
    • He named it "bicarburet of hydrogen."
  • It was synthesized in 1834 by Eilhard Mitscherlich.
    • He determined the molecular formula to be C6H6.
    • He named it benzin.
  • Early researchers classified these compounds as aromatic due to their pleasant smell and low C:H ratios.

Kekulé Structure

  • Proposed in 1866 by Friedrich KekulĂ© shortly after the concept of multiple bonds was introduced.
  • Failed to explain the existence of only one isomer of 1,2-dichlorobenzene.

Resonance Structures of Benzene

  • Benzene is a resonance hybrid between two KekulĂ© structures.
  • The carbon-carbon bond lengths in benzene are shorter than typical single-bond lengths but longer than typical double-bond lengths, giving it a bond order of 1.5.
  • Resonance can be represented by drawing a circle inside the six-membered ring.

Structure of Benzene

  • Each carbon atom is sp^2 hybridized with an unhybridized p orbital perpendicular to the ring.
  • These p orbitals overlap around the ring, resulting in six pi electrons delocalized over the six carbon atoms.

Unusual Addition of Bromine to Benzene

  • Bromine addition to benzene requires a catalyst like FeBr_3.
  • The reaction results in the substitution of a hydrogen atom by a bromine atom.
  • Direct addition of Br_2 to the double bond is not observed.

Resonance Energy

  • Benzene's heat of hydrogenation is less negative than predicted.
  • Predicted heat of hydrogenation: -359 kJ/mol.
  • Observed heat of hydrogenation: -208 kJ/mol.
  • The difference ( 151 kJ/mol ) is the resonance energy.

Molar Heats of Hydrogenation

  • Cyclohexene: -120 kJ/mol
  • Cyclohexadiene: -232 kJ/mol (-240 kJ/mol predicted)
  • Benzene: -208 kJ/mol (-359 kJ/mol predicted) with a resonance energy of 151 kJ/mol
    • The resonance energy > 8 kJ/mol

Annulenes

  • Annulenes are cyclic hydrocarbons with alternating single and double bonds.
  • Benzene is a six-membered annulene, named [6]-annulene.
    • Cyclobutadiene is [4]-annulene.
    • Cyclooctatetraene is [8]-annulene.

Failures of the Resonance Picture

  • Early theory suggested all cyclic conjugated hydrocarbons are aromatic.
  • Cyclobutadiene is highly reactive and dimerizes before isolation.
  • Cyclooctatetraene readily adds Br_2 to its double bonds.

MO Rules for Benzene

  • Six overlapping p orbitals form six molecular orbitals (MOs).
    • Three are bonding, and three are antibonding.
  • The lowest-energy MO has all bonding interactions and no nodes.
  • As energy increases, the number of nodes increases.

MOs for Benzene

  • \pi_1: All bonding
  • \pi2 and \pi3: Bonding with one node each
  • \pi4* and \pi5*: Antibonding with two nodes each
  • \pi_6*: All antibonding

First MO of Benzene

  • The first MO of benzene is entirely bonding with no nodes.
  • It has very low energy due to six bonding interactions and electron delocalization.

Intermediate MO of Benzene

  • Intermediate levels are degenerate (equal in energy).
  • \pi2 and \pi3 each have one nodal plane.

All-Antibonding MOs of Benzene

  • The all-antibonding \pi_6* has three nodal planes.
  • Each adjacent p orbital pair is out of phase and interacts destructively.

Energy Diagram for Benzene

  • Six p electrons fill the three bonding pi orbitals.
  • All bonding orbitals are filled ("closed shell"), creating an extremely stable arrangement.

MOs for Cyclobutadiene

  • \pi_1: All bonding
  • \pi2 and \pi3: One bonding, one antibonding
  • \pi_4*: All antibonding

Electronic Energy Diagram for Cyclobutadiene

  • Following Hund's rule, two electrons occupy separate, nonbonding molecular orbitals.
  • This diradical is highly reactive.

Polygon Rule

  • The energy diagram for an annulene has the same shape as the cyclic compound with one vertex at the bottom.

Aromatic Requirements

  • Cyclic structure with conjugated pi bonds.
  • Each ring atom must have an unhybridized p orbital (sp^2 or sp).
  • Continuous p orbital overlap around the ring.
  • Planar structure for effective overlap.
  • Delocalization of pi electrons must lower electronic energy.

Nonaromatic Compounds

  • Lack a continuous ring of overlapping p orbitals.
  • May be nonplanar.

Antiaromatic Compounds

  • Cyclic and conjugated with overlapping p orbitals, but electron delocalization increases electronic energy.

Hückel’s Rule

  • For a cyclic molecule with continuous overlapping p orbitals:
    • Aromatic if the number of pi electrons is (4N + 2), where N is an integer.
    • Antiaromatic if the number of pi electrons is (4N), where N is an integer.

Orbital Overlap of Cyclooctatetraene

  • Cyclooctatetraene assumes a nonplanar tub conformation, avoiding overlap between adjacent pi bonds.
  • HĂĽckel's rule does not apply.

Annulenes

  • [4]Annulene (cyclobutadiene) is antiaromatic.
  • [8]Annulene (cyclooctatetraene) would be antiaromatic, but it's nonplanar, so it's nonaromatic.
  • [10]Annulene is aromatic except for nonplanar isomers.
  • Larger 4N annulenes are not antiaromatic due to flexibility and nonplanarity.

[10]Annulene

  • All-cis [10]annulene has excessive angle strain in a planar conformation.
  • The isomer with two trans double bonds cannot be planar due to hydrogen atom interference.

MO Derivation of Hückel’s Rule

  • Aromatic compounds have (4N + 2) electrons and filled orbitals.
  • Antiaromatic compounds have 4N electrons and unpaired electrons in two degenerate orbitals.

Cyclopentadienyl Ions

  • Cation: An empty p orbital and four pi electrons, so it is antiaromatic.
  • Anion: A nonbonding pair of electrons in a p orbital for six pi electrons, so it is aromatic.

Deprotonation of Cyclopentadiene

  • Deprotonation of the sp^3 carbon creates a pair of electrons in one of the sp^3 orbitals.
  • This sp^3 orbital can rehybridize to a p orbital.
  • The six electrons in the p orbitals delocalize over all five carbon atoms, making the compound aromatic.

Orbital View of the Deprotonation of Cyclopentadiene

  • Deprotonation allows overlap of all p orbitals.
  • Cyclopentadiene is less stable than benzene and reacts readily with electrophiles.

Cyclopentadienyl Cation

  • HĂĽckel’s rule predicts that the cyclopentadienyl cation, with four pi electrons, is antiaromatic.
  • Consistent with this, the cyclopentadienyl cation is not easily formed.

Cycloheptatrienyl Cation

  • The cycloheptatrienyl cation has six pi electrons and an empty p orbital.
  • It's easily formed by treating the corresponding alcohol with dilute (0.01N) aqueous sulfuric acid.
  • Commonly known as the tropylium ion.

Cyclooctatetraene Dianion

  • Cyclooctatetraene reacts with potassium metal to form an aromatic dianion.
  • The dianion has ten pi electrons and is aromatic.

Pyridine Pi System

  • Pyridine has six delocalized electrons in its pi system.
  • Two nonbonding electrons on nitrogen are in an sp^2 orbital and do not interact with the pi electrons of the ring.

Pyridine

  • Pyridine is basic, with a pair of nonbonding electrons available to abstract a proton.
  • The protonated pyridine (the pyridinium ion) is still aromatic.

Pyrrole Pi System

  • The pyrrole nitrogen atom is sp^2 hybridized with a lone pair of electrons in the p orbital.
  • This p orbital overlaps with the p orbitals of the carbon atoms to form a continuous ring.
  • Pyrrole is aromatic because it has six pi electrons (N = 1).

Pyrrole

  • Pyrrole is aromatic because the lone pair on nitrogen is delocalized.
  • N-protonated pyrrole is nonaromatic because the nitrogen is sp^3 hybridized.

Basic or Nonbasic?

  • Pyrimidine has two basic nitrogens.
  • Imidazole has one basic nitrogen and one nonbasic.
  • Only one of purine’s nitrogens is basic.

Other Heterocyclics

  • Cyclopentadienyl anion (six pi electrons)
  • Pyrrole (six pi electrons)
  • Furan (six pi electrons)
  • Thiophene (six pi electrons)

Polynuclear Aromatic Hydrocarbons

  • Naphthalene
  • Anthracene
  • Phenanthrene

Naphthalene

  • Fused rings share two atoms and the bond between them.
  • Naphthalene is the simplest fused aromatic hydrocarbon.

Larger Polynuclear Aromatic Hydrocarbons

  • Formed in combustion (tobacco smoke).
  • Many are carcinogenic.
  • Epoxides form and combine with DNA bases leading to mutations.
  • Examples: benzo[a]pyrene.

Allotropes of Carbon

  • Amorphous: Small particles of graphite; charcoal, soot, coal, carbon black
  • Diamond: A lattice of tetrahedral carbon atoms
  • Graphite: Layers of fused aromatic rings

Diamond

  • One giant molecule
  • Tetrahedral carbon.
  • Sigma bonds, 1.54 Ă…
  • Electrical insulator

Graphite

  • Planar layered structure
  • Layer of fused benzene rings, bonds: 1.415 Ă…
  • Only van der Waals forces between layers
  • Conducts electrical current parallel to layers

Some New Allotropes

  • Fullerenes: Five- and six-membered rings arranged to form a “soccer ball” structure
  • Nanotubes: Half of a C_{60} sphere fused to a cylinder of fused aromatic rings

Fused Heterocyclic Compounds

  • Purine
  • Indole
  • Benzimidazole
  • Quinoline
  • Benzofuran
  • Benzothiophene
  • Examples from nature and drugs: L-tryptophan, benziodarone, LSD, quinine

Common Names of Benzene Derivatives

  • Phenol (benzenol)
  • Toluene (methylbenzene)
  • Aniline (benzenamine)
  • Anisole (methoxybenzene)
  • Styrene (vinylbenzene)
  • Acetophenone (methyl phenyl ketone)
  • Benzaldehyde
  • Benzoic acid

Disubstituted Benzenes

  • Numbers identify the relationship between groups.
    • Ortho- (o-) is 1,2-disubstituted.
    • Meta- (m-) is 1,3-disubstituted.
    • Para- (p-) is 1,4-disubstituted.

Three or More Substituents

  • Use the smallest possible numbers.
  • The carbon with a functional group is number 1.

Common Names for Disubstituted Benzenes

  • m-xylene (1,3-dimethylbenzene)
  • Mesitylene (1,3,5-trimethylbenzene)
  • o-toluic acid (2-methylbenzoic acid)
  • p-cresol (4-methylphenol)

Phenyl and Benzyl

  • Phenyl indicates the benzene ring attachment.
  • The benzyl group has an additional carbon.

Physical Properties of Aromatic Compounds

  • Melting points: More symmetrical than corresponding alkanes, pack better into crystals, giving higher melting points.
  • Boiling points: Dependent on dipole moment, so ortho > meta > para for disubstituted benzenes.
  • Density: More dense than nonaromatics, less dense than water.
  • Solubility: Generally insoluble in water.

Physical Properties of Benzene Derivatives

  • Examples provided with melting points, boiling points, and densities.

IR and NMR Spectroscopy

  • C=C stretching absorption at 1600 cm^{-1}.
  • sp^2 C—H stretch just above 3000 cm^{-1}.
  • ^1H NMR at δ 7–δ 8 for H’s on aromatic ring
  • ^{13}C NMR at δ 120–δ 150, similar to alkene carbons

Mass Spectrometry

  • The benzylic position is prone to fragmentation, leading to a benzyl cation (m/z = 91), which can rearrange to the tropylium ion.

UV Spectroscopy

  • Benzene has a moderate band at 204 nm and a benzenoid band at 254 nm.

Ultraviolet Spectra of Benzene and Some Derivatives

  • Comparison of UV spectra for benzene, ethylbenzene, m-xylene, bromobenzene, and styrene.